1. Field of the Invention
The present invention relates to an applicator and method for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
Applicators of the type described above are used in conjunction with so-called coaters to provide one or both sides of a traveling material web consisting, e.g., of paper, cardboard or textile material, with one or several layers of a coating medium, for example, dye, starch, impregnating fluid or the like.
In the so-called direct application, the liquid or pasty coating medium is applied by an applicating apparatus directly onto the surface of the traveling material web, which during application is carried on a revolving backing surface, for example, an endless belt or a backing roll. In the indirect application of the medium, in contrast, the liquid or pasty applicating medium is first applied onto a substrate surface, e.g., the surface of a backing roll fashioned as an applicator roll, and is transferred there, in a nip through which the material is passed, from the applicator roll to the material web.
Categorical applicators are known from DE-PS-4425137, DE-OS-3438181 and U.S. Pat. No. 5,435,847 and include a backing roll, on which travels a material web to be provided with a liquid or pasty coating medium. Also included are an applicating apparatus opposing the backing roll and serving to apply the coating medium, and a doctor element following the applicating apparatus and extending across the entire machine width, for example, a doctor blade, which removes surplus applied coating medium. Furthermore, such applicators include at least one adjustable coating edge delimiting apparatus for setting the coating width on an applicator roll, backing roll or the traveling material web.
The applicator according DE-PS 4425137 possesses as applicating apparatus a coating chamber formed between a slice and the doctor element, which chamber is sideways sealed by sealing blocks. Employed as coating edge delimiting apparatus are two deckle plates which are coordinated with the side edges of the traveling material web and each retained by a deckle plate holder joined to the respective sealing block. A coating edge delimiting element formed thereby, for adjustment of the coating format, is movable by means of a sliding element in a direction transverse to the traveling direction of the material web, and remains during the operation of the applicator in the position once set.
The applicator according to DE-OS 3438181 resembles substantially that of DE-PS-4425137, but possesses coating edge delimiting apparatus elements that are provided separately from the lateral deckle blocks or deckle plates of the coating chamber and allow setting and locking relative to the backing roll. More exactly, these coating edge delimiting elements are so-called edge scrapers, which, analogously to the deckle plate according to DE-PS 4425137, remain during the operation of the applicator in the position once set.
The applicator according to U.S. Pat. No. 5,435,847 also corresponds substantially to those pursuant to DE-OS-3438181 and DE-PS-4425137. But it possesses as coating edge delimiting apparatus two deckle slides that are each associated with a side edge of the traveling material web. The deckle slides seal, sideways, a gap formed between two opposing walls. Each deckle slide is for adjustment within the gap of the coating width on the traveling material web in a direction transverse to the traveling direction of the material web. Each deckle slide can be fixed and remains during the operation of the applicator in the position once set.
These prior applicators have various disadvantages, which will be more fully explained now with reference to the attached FIG. 3. Shown in FIG. 3 is a schematic plan view of a prior-art applicator in the area of the doctor element and of the material web edge, with the doctor element, here, a doctor blade 14, folded over in the plane of the material web, for better illustration. Already coated with a liquid or pasty coating medium 4 by means of a not illustrated applicating apparatus, the material web 6 with a width B.sub.M is carried on a backing roll 2 having a width B.sub.W. The areas of the material web 6 coated with the pasty coating medium 4 are illustrated stippled in FIG. 3. As shown in FIG. 3, a faster and greater local wear 24 of doctor element 14 and sort of a scoring effect occurs at the transition between the coated B.sub.G and uncoated (edge) area B.sub.R. The service life of doctor element 14 is considerably reduced thereby, requiring more frequent replacement of this component, greater cumulative downtime of the applicator, and, consequently, higher operating costs. Due to the aforementioned point of wear 24, doctor element 14 is normally no longer usable even with a format adjustment modified relative to the original setting, that is, with a modified coating width B.sub.G, since wear point 24 may lead to an undesirable coating irregularity and quality impairment of the finished product. Hence, doctor element 14 normally must be replaced by another. When the transition between coated B.sub.G, and uncoated edge area B.sub.R is located not on the applicator roll or backing roll 2, but on material web 4 itself, considerable problems in winding, unwinding or rewinding of material web 4 may additionally occur, due to the abrupt thickness difference of material web 4 resulting therefrom, i.e., the thickness difference between the coated B.sub.G, and uncoated B.sub.R material web section.